DVS sensors are cameras in which each pixel generates an event every time the light striking it changes in a fixed ratio since said pixel generated the previous event. If light increases, the event will be positive and it will be negative if the light fades. This way, the sensor generates a flow of events over time, where each event is defined by the three components (x, y, s), wherein (x, y) are pixel coordinates in the matrix and ‘s’ is the event sign. This flow of events represents the changing visual scene that is captured by the sensor. This sensor concept was originally introduced by Kramer ((J. Kramer, “An Integrated Optical Transient Sensor,” IEEE Transactions on Circuits and Systems, Part-II: Analog and Digital Signal Processing, vol. 49, No. 9, pp. 612-628, September 2002) and (J. Kramer, “An on/off transient imager with event-driven, asynchronous read-out,” IEEE Int. Symp. On Circuits and Systems, ISCAS 2002, vol. II, pp. 165-168, 2002)), but its embodiment posed a severe mismatch in pixel performance, which limited the maximum temporal contrast sensitivity that could reach values of about 30% (P. Lichtsteiner, et al “Improved ON/OFF Temporally Differentiating Address-Event Imager,” Proceedings of the 2004 IIth IEEE International Conference on Electronics, Circuits and Systems, 2004. ICECS 2004, pp. 211-214). In order to improve this state of the art, Lichtsteiner subsequently proposed an improved sensor by introducing a self-timing switched capacitor stage with two capacitors (U.S. Pat. No. 5,168,461) providing a lower mismatch in the performance from pixel to pixel, thus making possible to achieve sensibilities to the temporal contrast in the range of 15% ((P. Lichtsteiner, et al, “A 128×128 120 dB 15 μs Latency Asynchronous Temporal Contrast Vision Sensor,” IEEE J. Solid-State Circ., vol. 43, No. 2, pp. 566-576, February 2008) and (U.S. Pat. No. 7,728,269 B2)).
However, the switched capacitor stage required that the two capacitors have a very disparate value, which in one embodiment of integrated circuit results in the requirement of a considerable area within each pixel area. In the sensor manufactured by Lichtsteiner ((P. Lichtsteiner, et al, “A 128×128 120 dB 15 μs Latency Asynchronous Temporal Contrast Vision Sensor,” IEEE J. Solid-State Circ., vol. 43, No. 2, pp. 566-576, February 2008) and (U.S. Pat. No. 7,728,269 B2)) these capacitors took approximately two thirds of the total area of the pixel. Therefore, as pixels are large, the chip occupies a large area and is expensive. In order to improve this new state of the art, Leñero (J. A. Leñero-Bardallo, et al, “A 3.6 us Asynchronous Frame-Free Event-Driven Dynamic-Vision-Sensor,” IEEE J. of Solid-State Circuits, vol. 46, No. 6, pp. 1443-1455, June 2011) proposed reducing the disparity between the value of the capacitors while introducing a voltage amplifier stage of very small area before that of the switched capacitators, thus achieving both the reduction of the area of the pixel and slightly improving temporal contrast sensitivity up to values of about 10%. However, this amplifier stage had a high consumption and slightly deteriorated the mismatch of pixel performance.
To explain the improvement achieved by the present invention over the state of the art, Lichtsteiner's sensor (U.S. Pat. No. 7,728,269 B2) has been taken as reference, whose pixel simplified diagram is shown in FIG. 1. The light sensed by photodiode D is transformed into photocurrent Iph. The transistors T1 to T4 logarithmically transforms Iph into the voltage VP1=Voffset+V0 log (Iph) in the node P1. The photocurrent Iph, which flows through the transistor T4 and exits from the drain node P0, which is shared by all pixels of the matrix, is added in the current adder block ΣI, which also ads the photocurrents from all pixels in the matrix. This sum is subsequently used to automatically adjust the gate of the transistor T3 in the pixels to minimize consumption of the amplifier T1-T3 to adapt it to ambient light (US 2004/065876). The transistors T5a and T5b copy VP1 to node P2. In Leñero's improvement, these two transistors are replaced by a voltage amplifier stage with gain Av, so that the voltage at P2 would be VP2=Av (Voffset+V0 log (Iph)), wherein Av=1 for the embodiment according to Lichtsteiner and AV>1 for the embodiment according to Leñero. The switched capacitor circuit comprising the capacitors C1 and C2 and the transistors T6 to T8, copy the voltage variation at P3 to P2 from a previous reset time t1 multiplied by the capacitive gain Ac=C2/C1. Thus, VP3(t)=Ac(VP2(t)−VP2(t1))=AcAvV0 log (Iph (t)/Iph (t1)). The transistors T9 to T11 detect whether VP3 (t) exceeds a specific positive threshold VR+ and if so, they generate a positive event (ON). The transistors T12 to T14 detect whether VP3(t) falls below a negative threshold VR− and if so, they generate a negative event (OFF). Every time the pixel generates an event, a reset of capacitor C1 occurs by means of the reset transistor T7. This way, the pixel immediately generates a positive event t2 if VP3(t2)≧VR+=AcAvV0 log (Iph(t2)/Iph(t1)), and a negative event if VP3(t2)≦VR−=AcAvV0 log (Iph(t2)/Iph(t1)). This can also be expressed as ΔI/I=exp ((VR+/−)/(AcAvV0))−1=θ+/−. Where the parameter θ+/− represents the sensitivity to the positive or negative contrast. The minimum value that can be adjusted for this contrast sensitivity is given by the dispersion from pixel to pixel of the parameters VR+/−, Ac, Av and V0. The parameter V0 is usually a function of physical constants and does not undergo dispersion from pixel to pixel in the same chip. The dispersion of the parameters VR+/− is given by the dispersion in the performance of the amplifiers T6 and T8 and the voltage comparators (transistors T9 to T11 and T12 to T14) and is normally high because the amplifiers T6 and T8 and the comparators are made small to reduce the total pixel area. The impact of high mismatch of the amplifier and comparators is reduced by increasing the product of the denominator AcAv. In Lichtsteiner's state of the art Av=1, whereby it was mandatory to make Ac as big as possible. For example, in the Lichtsteiner's embodiment ((P. Lichtsteiner, et al, “A 128×128 120 dB 15 μs Latency Asynchronous Temporal Contrast Vision Sensor,” IEEE J. Solid-State Circ., vol. 43, No. 2, pp. 566-576, February 2008) a value of 20 was given. The parameter A, also undergoes dispersion from pixel to pixel, but it is reduced because in integrated circuits the relationship between capacitances is subject to low dispersion (typically below 1%). In Letiero's embodiment, the parameter Av also introduces dispersion. However, the parameter Ac could be reduced to 5 while Av was set at about 25. In this way, the product was 125, which improved overall contrast sensitivity in spite of slightly increasing the dispersion. However, the additional amplifier stage greatly increased the pixel consumption (above a factor of 10).
Therefore, the state of the art poses the problem that contrast sensitivity cannot be improved without increasing the pixel area or without increasing the power consumption. In order to solve the problems associated with the state of the art, the present invention uses transimpedance amplifiers by connecting MOS transistors, polarised in weak inversion and having a diode configuration, which are connected in series (ES 201130862).